This is WDW's achille's heal -- and I think they know it. WDW's strategy has been to keep people on property so as to capture 100% of their vacation spend - park, hotel, food, and merch. If a guest leaves property, even if only for a day or two, everything changes as to where that guest may spend its money. As @
ParentsOf4 has aptly pointed out many times, WDW is a hotel and timeshare businesses. An incredibly profitable one with exorbitant rates. To fill those rooms WDW operates theme parks and gives guests free transportation on property and to/from the airport. If a guest stays on property, Disney makes their lives easy (ok, well, not really but we'll debate the horrid state of the bus system later)
BUT, leaving property adds a new level of complication to the vacation plans. If a guest goes to Universal for a day or two, now they need a car (easiest way). Once a guest has a car, WDW's walled garden begins to fall apart. I completely agree that maybe in the short term the guest will continue to stay on property, but as more and more spend time off property, the perceived benefit of staying on property begins to quickly diminish, especially considering it is markedly more expensive, and maybe next trip the guest will go off property.
Furthermore, the profitability of that guest begins to drop rapidly too. Disney doesn't care about losing the park days -- afterall a 5 day ticket only costs a few dollars more than a 4 day ticket -- but is very concerned about losing nights spent at its hotels, as each night costs the same. As far as Disney is concerned, the fewer days a guest spends at the parks, the better, as the parks cost money to operate. I don't think Disney is concerned that Uni is building amazing theme parks that will draw guests for a day or two -- it welcomes that. But, I think Disney should be very concerned that Uni plans to significantly increase its hotel offerings as well. And, Uni is also building compelling reasons to consider its hotels, which are considerably less expensive that Disney's.